1. Field of the Invention
This invention relates to the field of alarm systems, and in particular to a means of diagnosing and indicating the status of a plurality of connected sensing contacts, and which of the contacts may be causing an alarm-generating condition.
2. Description of the Prior Art
Alarm systems based upon normally-closed contacts wired in series are well known. The contacts are usually mounted at locations which will be encountered by an intruder or other factor generating an alarm condition. For example, normally-closed limit switches may be serially wired and placed around doors and windows to be secured, opening of any of the doors and windows causing a cessation of current in the alarm network, and generating an alarm. Similar switch apparatus may be mounted on interior doors, under rugs, or may be associated with the relay contacts of light beam and photo cell sensors.
Alarm systems are preferably based upon normally-closed contacts wired in series, rather than normally-open contacts wired in parallel. In this manner, a fault in the wiring connecting the switches, or the deliberate breaking of a connection by an intruder, will generate an alarm condition just as if switch contacts were closed. Moreover, such normally-closed contacts can be combined with easily-broken conductors, for example, metallic tape on windows, fusible conductors for detecting fires, and the like.
A drawback of normally-closed switch and conductor systems is that they require a constant supply of current. In the event that the installation requires protection during a time of disconnection of power, a battery is necessary. Therefore, there may be situations in which a parallel-contact installation or a combination of series and parallel sensors is deemed necessary.
For the most part, prior art systems have been based upon series-wired switches connected to a single annunciator, i.e., an electrically controlled apparatus for producing audible and/or visible signals. The system is thus applicable to produce an alarm condition when any of the series-wired switches is open. A system according to the foregoing description, however, does not indicate which of the plurality of connected switches is the source of the open circuit. The alarm condition may be due to more than one of the switches being open. It is possible to wire both leads of each normally-closed switch to a central location whereby they can be individually tested by sequentially (or simultaneously) attempting to pass a current through the individual switch contacts. The present invention provides the benefits of such a complicated individually-addressable-contact network, in a much simpler way.
According to the invention, the individual contacts are wired in a conventional system in series with one another and with an annunciator. The annunciator is a direct-current-powered device which may operate, for example, by disabling an audible alarm so long as power is applied to the annunciator through the series connected electrical contacts. Each contact is supplied according to the invention with a reverse-biased light-emitting diode ("LED"), and a switch network is operable in a test mode to place a test voltage, at a polarity opposite that of the annunciator in its normal operating mode, across the series-connected contacts. Therefore, any of the contacts which remain open will be indicated by the actuation of the associated LED.
The system according to the invention does not require that the LED indicators for the individual electrical contacts by located at any central location, or be individually tested, although they may be centrally located if deemed advisable. Additional indicators, including a "test condition" indicator in series with the test voltage supply, and a ground fault indicator, running from ground to the test voltage supply, are provided.
The invention is applicable to either an original installation or a retrofit installation with an existing alarm system characterized by series-connected normally-closed switches. In a retrofit, the user need only wire the device of the invention between the annunciator and the series-connected contacts, and connect LED indicators in parallel with the individual contacts as needed. In the event less than all of the series-connected contacts are provided with indicators, the "test condition" indicator is operable to localize the open circuit condition to the portion of the network which does not have indicators.